Ok.
We think everything in the instrument cluster is disconnected, the front body harness is disconnected and the rear body harness is disconnected so those should be out of the picture. You do have this rear body harness disconnected, right?
You mentioned that you have to have the head light switch plugged in in order to make it blow the fuse. I assume from that statement that you have tried putting a new fuse in with the head light switch unplugged and it didn't blow. Right?
You said earlier "On the back of the headlight switch there are three prongs in a line but on the actual harness itself there are only two. is this that way its supposed to be?" That has me puzzled because, according to the wiring schematic in the 78 factory chassis manual, there are 6 wires connected to the head light switch. There does appear to be three pins in a line but only two of them have wires going to them. The other 4 pins are situated differently.
1. There's a
red wire which actually has to travel from the 20 amp "TAIL" fuse block out into the engine compartment, go through a fusable link and then come back inside the body and finally connect to the headlight switch. I know the bulkhead connector has two separate halves. I assume you still have the correct one plugged in or you wouldn't be getting any voltage on the red wire at the switch. This makes sense because the other half of the bulkhead connector houses the wires that go to the parking/marker lights. That has to be the one you have disconnected.
2. Also attached to the switch is a
green wire that comes from the "LPS" fuse in the fuse block on the very bottom near the center. That fuse isn't blowing so that
probably isn't the problem.
3. There's a
brown wire that comes a spot in the fuse block only labeled "41". That fuse isn't blowing so that one
probably isn't the problem.4. There are
two brown wires that are tied together at the head light switch connector. One goes to the digital clock. The other brown wire goes to the bulkhead plug where it is tied with yet another brown wire. One goes through the bulkhead connector and connects to the front parking and marker lights and the other brown wire go to the rear and connects to the rear license, tail and marker lights. The problem could be in one of these wires. I am not sure of that digital clock info in the book. It prbably refers to the digital clock that was in the digital stereo. You don't have the digital stereo do you? If not, those wires are probably not connected to anything which could mean
this wire could be the problem. The standard clock is fed from the "CLK LTR CTSY" fuse.
5. There's a
light blue wire that goes to the dimmer switch. Kinda
doubt this would be the problem. The schematic doesn't show the internal connections of the head light switch but I don't think the dimmer gets connected until the 2nd click when the head lights come on.
6. That leaves us with the white wire which connects to the door jamb switches to provide a ground path when the doors are opened or the head light knob is turned fully counter clockwise. You said this is all working correctly so that
probably isn't the problem.I would try disconnecting all but the red wire from the head light switch and put a fuse in and pull the knob. If it doesn't blow, reinstall one wire at a time until it blows. Then, hopefully, you'll be down to one wire to trace. You can use a small flat head screw driver to get the pins out of the plug. Make sure to write down where each pin goes.
Hey, before you try this, unplug the turn signal connector and try it then. It's the long skinny plug that mounts to the column itself. Should be 11 pins in a single row. I don't think you've tried that yet and it would be easy.
It could be that the problem is actually behind the fuse block. Someone in the past may have taken it out for whatever reason and pinched a wire when they put it back in. You might pull it off and have a look at the wires where they go into the block.
I know this sounds like a huge pain. It would be much easier with a meter.
Good luck,
